\documentclass[UTF8,12pt]{article}
\usepackage{ctex}
\usepackage{changepage}
% \usepackage{indentfirst} 
\begin{document}
\section{Paragraph Indentation 1}

LaTeX is a high-quality typesetting system; it includes features 
designed for the production of technical and scientific documentation. 
LaTeX is the de facto standard for the communication and publication of 
scientific documents. LaTeX is available as free software.

LaTeX is a high-quality typesetting system.

LaTeX is a high-quality typesetting system.

\section{Paragraph Indention 2}
\setlength\parindent{1em}

LaTeX is a high-quality typesetting system; it includes features 
designed for the production of technical and scientific documentation. 
LaTeX is the de facto standard for the communication and publication of 
scientific documents. LaTeX is available as free software.

\setlength\parindent{5em}
\noindent LaTeX is a high-quality typesetting system.

LaTeX is a high-quality typesetting system.

LaTeX is a high-quality typesetting system.

\setlength\parindent{3em}
LaTeX is a high-quality typesetting system.

\setlength\parindent{2em}
LaTeX is a high-quality typesetting system.

%\noindent
\setlength\parindent{0em}
\hangafter=0
\setlength{\hangindent}{2em}
Hangafter LaTeX is a high-quality typesetting system; it includes features 
designed for the production of technical and scientific documentation. 
LaTeX is the de facto standard for the communication and publication of 
scientific documents. LaTeX is available as free software.
LaTeX is a high-quality typesetting system; it includes features 
designed for the production of technical and scientific documentation. 
LaTeX is the de facto standard for the communication and publication of 
scientific documents. LaTeX is available as free software.

\hangafter 1
\hangindent 1.5em
Hangafter LaTeX is a high-quality typesetting system; it includes features 
designed for the production of technical and scientific documentation. 
LaTeX is the de facto standard for the communication and publication of 
scientific documents. LaTeX is available as free software.
LaTeX is a high-quality typesetting system; it includes features 
designed for the production of technical and scientific documentation. 
LaTeX is the de facto standard for the communication and publication of 
scientific documents. LaTeX is available as free software.


结论
\begin{itemize}
	\item 悬挂缩进就是整体段落开始空出的距离。
	\item 悬挂缩进可以从第1行(设置值从0开始)至第n行在整个段落的左侧都可以留出指定长度的空白。
	\item 当开始值为0，即从第1行开始时，第1行同时受到段落缩进和悬挂缩进的影响，此时悬挂缩进起作用。
	如上所示，当我们将段落缩进设置为0em和悬挂缩进设置为2em时，缩进为2em。
	\item $\backslash$hangafter=0 可以用 $\backslash$hangafter 1 简化。
	\item $\backslash$setlength$\backslash$parindent\{2em\} 可以用 $\backslash$hangindent 2em 代替。
\end{itemize}


\begin{adjustwidth}{2cm}{1cm}
	\hangafter 1
	\hangindent 1.5em
	\setlength\parindent{10em}
	\textsc{
	\qquad Hangafter LaTeX is a high-quality typesetting system; it includes features 
	designed for the production of technical and scientific documentation. 
	LaTeX is the de facto standard for the communication and publication of 
	scientific documents. LaTeX is available as free software.
	LaTeX is a high-quality typesetting system; it includes features 
	designed for the production of technical and scientific documentation. 
	LaTeX is the de facto standard for the communication and publication of 
	scientific documents. LaTeX is available as free software.}
\end{adjustwidth}


结论
\begin{itemize}
	\item 使用adjustwidth以后，前面的缩进命令不再起作用；
	\item 首先缩进可以使用 $\backslash$qquad 实现。
\end{itemize}


\end{document}